Christos Katsavrias

Radial diffusion, driven by ultra‐low frequency (ULF) waves, is an important process for the transport of electrons in the outer radiation belt. Ground magnetometers give us constant observations of such ULF waves but their usefulness is limited by the models used to transform ground measurements into their progenitor fields in space. A typical assumption is that of a dipole field, and so measurements can only be reliably performed during the day. We have chosen a series of 17 magnetometer stations at different sets of geomagnetic latitudes, with stations in each set separated by several hours in longitude, and compared their Pc5 (1.67–6.67 mHz) ULF power measurements, and the resulting calculated radial diffusion coefficients, from 11 years of their data. By comparing the mismatch of their results when they were in different time sectors, and for different values of Kp, we were able to assess the median deviation of measurements conducted before dawn or after dusk with those on the day side. This will be useful for projects requiring a longitudinally limited array of magnetometers, such as FARBES (Forecast of Actionable Radiation Belt Scenarios), or for parts of the world where such coverage is limited.

The geomagnetic superstorm in May 2024 represents the most extreme space weather event over the past two decades, offering a unique opportunity to investigate radiation belt electron dynamics under exceptionally strong solar wind driving conditions. Observations from the Arase satellite show that relativistic electron fluxes dropped by several orders of magnitude during the storm main phase. Although magnetopause shadowing and wave‐driven losses are established mechanisms, the response of radiation belt electrons to superstorm‐level solar wind driving remains poorly constrained. Using the Versatile Electron Radiation Belt model, we present the first physics‐based simulation of the May 2024 electron dropout. Utilizing multi‐satellite and ground‐based observations, we show that the decades‐long, widely adopted Kp‐driven radial diffusion parameterization fails to represent radial transport under superstorm conditions, substantially misrepresenting both the timing and magnitude of enhanced diffusion relative to observed ultra‐low‐frequency wave activity. Accurate modeling of the observed extreme dropout requires precise specification of the onset of enhanced radial diffusion, which should coincide with the nearly simultaneous storm‐time magnetopause compression. These coupled processes drive the dramatic depletion, followed by scattering from whistler‐mode plasma waves.

Context. Quasi-periodic density structures (PDSs) are quasiperiodic variations in the solar wind density that range from a few minutes to a few hours. They are trains of advected density structures with radial length scales L _R ≈ 100 − 10 000 Mm, and they thus belong to the class of so-called mesoscale structures in the solar wind. Although the precise mechanisms that form PDSs are still debated, cumulative evidence from multiple studies using in situ and remote data supports the view that most PDSs have a solar origin and do not form through dynamics during their propagation in interplanetary space. Low-frequency (< 1 mHz) PDSs have been associated with small-scale flux ropes, which further indicates a solar origin. Aims. We further investigated the origin and properties of PDSs by searching for coherent small-scale helical structures and flux ropes within PDS intervals. Methods. We used an extensive list of PDSs, compiled from Solar Orbiter measurements, and we applied a wavelet analysis technique to obtain the reduced magnetic helicity, cross helicity, and residual energy. Results. Our results indicate that small flux ropes are a constituent of PDS events, while the occurrence probability of helical Alfvénic structures is similar within and outside the PDSs. Conclusions. These results are consistent with the scenario in which PDSs are formed by processes involving magnetic reconnection.

The geomagnetic storm in May 2024 represents the most extreme space weather event over the past twenty years, providing a unique opportunity to investigate energetic particle dynamics under exceptionally strong solar wind driving conditions. Among the various particle populations affected by such extreme storms, outer radiation belt electrons are of particular interest because they respond rapidly to storm-time magnetospheric reconfigurations and pose significant hazards to satellites operating in near-Earth space.Observations from the Arase satellite reveal that MeV radiation belt electron fluxes dropped by several orders of magnitude during the main phase of the May 2024 superstorm. This extreme dropout coincided with intense magnetopause compression to a minimum standoff distance of L~3.7, estimated using the Space Weather Modeling Framework, and enhanced ultra-low-frequency (ULF) Pc5 wave activity derived from SuperMAG. To investigate the physical mechanisms controlling this extreme electron loss, we performed simulations using the Versatile near‐Earth environment of Radiations Belts and ring current (VERB) model, systematically examining the roles of magnetopause shadowing, radial diffusion, and local wave-driven scattering.We find that commonly used empirical radial diffusion models, parameterized by Kp, fail to reproduce the observed electron flux profiles during this event. Instead, accurately capturing the extreme electron dropout requires that enhanced radial diffusion be properly timed relative to magnetopause compression and storm-time wave activity. In addition, stronger plasmaspheric hiss scattering is necessary to reproduce losses at lower L*. These results demonstrate that extreme radiation belt electron dropout during superstorms is controlled by the coupled timing of magnetopause compression, radial diffusion, and local scattering processes, emphasizing the importance of physically timed transport and loss representations in extreme storm modeling.

The quasi-Periodic density structures (PDSs) are quasiperiodic variations of solar wind density ranging from a few minutes to a few hours. They are trains of advected density structures with radial length scales LR in the 100-10,000 Mm range, thus belonging to the class of solar wind mesoscale structures. Even though PDS at L1 have been extensively studied both through statistical and event analysis, their investigation at distances closer to the Sun is limited. This study performs a statistical investigation of PDS at various distances from the Sun between 0.3 and 1 AU by exploiting Solar Orbiter data. We compiled and made publicly available an extensive list of PDSs following a well-established methodology that combines the Multitaper method as well as wavelet analysis to reveal the distribution of PDS radial length scales and how they vary with respect to the radial distance. Our results indicate that PDS advected with the ambient slow solar wind are expanded at a rate of approximately 10%, while PDS detected during fast solar wind segments show compression at a similar rate. These are consistent with the scenario in which PDSs are formed at the Sun by processes involving magnetic reconnection and interchange reconnection in the solar corona.

Context. Stream interaction regions (SIRs) are large-scale solar wind structures that play a significant role in space weather in the near-Earth environment. Understanding their thermodynamic behaviour is essential to explaining the energy transfer processes governing their evolution. Aims. We investigate in depth (for the first time to such an extent) the behaviour of both the total and partial proton polytropic indices in SIRs and the high-speed streams (HSSs). Methods. To that end, we used a list of 186 SIRs identified from Wind measurements over more than two solar cycles (1995–2022), from which we derived the distributions of the polytropic index in the near-Earth space (L1). Results. Our results show that the slow compressed solar wind region has a sub-adiabatic polytropic index, which indicates compression and turbulent heating. The HSS exhibits a super-adiabatic polytropic behaviour, which is consistent with a decrease in the effective degrees of freedom and/or an additional energy release mechanism. We discuss the consistency of our findings with the fluctuating-moment effect in large-scale compressive fluctuations as such an energy release mechanism.

Context. Periodic density structures (PDSs) are a type of solar wind mesoscale structure characterised by quasi-periodic variations in the density of the solar wind ranging from a few minutes to a few hours. They are trains of advected density structures with radial length scales of L_R ≈ 100 − 10 000 Mm. Analysis of case studies shows that PDSs can be compressed when embedded in a stream interaction region (SIR), leading to larger density variations and an increased impact on the magnetospheric and radiation belt dynamics. Aims. We perform an extensive statistical study to identify PDSs embedded in SIRs as well as their corresponding frequency and radial length scale distributions. Methods. We used an extensive list of 186 SIRs and 1217 embedded PDS events from the entire Wind dataset (1995−2022), spanning more than two solar cycles, to investigate the frequency and radial length scales of PDSs. With the use of wavelet methods, we classified these PDSs as coherent or incoherent, based on the shared periodic behaviour between proton density and the alpha-to-proton ratio, and we derived the corresponding occurrence distributions. Results. We found that 130 out of 186 SIR events have embedded coherent PDSs, which exhibit an increasing probability of occurrence with increasing frequency (up to ≈3 mHz). Furthermore, the investigation of radial length scales of coherent PDSs in SIRs reveals significant compression compared to PDSs in the ambient solar wind, as the most probable L_R values are 120−130 Mm and 160−190 Mm for the slow and fast compressed solar wind, respectively. The coherent PDS L_R decreases with a rate of −0.74, while the corresponding amplitude increases with a rate of 0.74 with increasing solar wind proton density, both following a power law function. Conclusions. Our results indicate that coherent PDSs occur more often than not in SIRs. This is consistent with a picture in which PDSs are formed at the Sun, advected by the solar wind, and enhanced by their interaction with SIRs, while both their radial length scale and amplitude are controlled by the level of compression in the interaction region.

Motivated by the need for more accurate radiation environment modeling, this study focuses on identifying and analyzing the drivers behind the sub-relativistic electron flux variations in the inner magnetosphere. We utilize electron flux data between 1 and 500 keV from the Hope and MagEIS instruments on board the RBSP satellites, as well as from the FEEPS instruments on board the MMS spacecrafts, along with solar wind parameters and geomagnetic indices obtained from the OmniWeb2 and SuperMag data services. We calculate the correlation coefficients between these parameters and electron flux. Our analysis shows that substorm activity is a crucial driver of the source electron population (10–100 keV), while also showing that seed electrons (100–400 keV) are not purely driven by substorm events but also from enhanced convection/inward diffusion. By introducing time lags, we observed a delayed response of electron flux to changes in geospace conditions, and we identified specific time lag periods where the correlation is maximum. This work contributes to our broader understanding of the outer belt sub-relativistic electron dynamics and forms the basis for future research.

Radial diffusion in planetary radiation belts is a dominant transport mechanism resulting in the energization and losses of chargedparticles by large-scale electromagnetic fluctuations. In this work we exploit the extensive radial diffusion coefficients (DLL) database createdin the framework of the Horizon 2020 SafeSpace project, which spans 9 years of hourly DLL coefficients, to investigate the spatiotemporaldistributions of the coefficients. Our results indicate that the radial distribution of the magnetic and electric component of theDLL, as well as their sum, the total DLL, can be well described by a power law function of L* in the [4.3–7.7] range. We show that the L*-dependent spectral index varies significantly and is far from constant as assumed and implemented in many semi-empirical models. Weexamine the quasi-periodic behavior of the radial profiles of the DLL throughout most of the 24th Solar cycle, which the data cover, andfind an approximately 420 days dominant periodicity. This periodic behavior is linked (in terms of cross-wavelet analysis) with solaractivity, nevertheless, its origin remains unclear. The uncovered features are important for understanding DLL behavior and driversas well as for current and future modelling efforts.